A hologon is a device which, when rotated, causes a stationary beam directed at it to repetitively scan along a line. Hologons comprise a disc mounted for rotation at high speed. The disc has, on a planar surface to which the axis of rotation is perpendicular, a plurality of sector-shaped facets. Each facet contains a diffraction grating. If there are "n" facets, the inclination of grating lines in one facet to the lines in adjacent facets (360/n).degree.. The lines may be tangential or "radial". If "radial", the lines of a facet are parallel to a radius which bisects the facet. If tangential, the lines are perpendicular to the radius which bisects the facet. The gratings may be reflective or transmissive. Multifaceted hologons having "n" facets cause "n" scans per revolution.
It is known to make a hologon by coating a disc with photoresist or other photosensitive material such as dichromated gelatin. A radiation beam, usually a laser beam is split and the resulting two beams are interfered at the photoresist coated so that the coating is exposed to a rectilinear interference pattern. The extend of the latent image created is limited by a mask which has an aperture bounded in part by two radially inner and radially outer boundaries and opposed lateral boundaries radial to the axis of rotation of the disc. The angle included between this line is the same as the included angle of the facet, i.e., (360/n).degree.. After one facet has been exposed, the disc is rotated through an angle of (360/n).degree. relative to the mask and means for directing the radiation beams at the disc, and another facet is exposed. After all "n" facets have been exposed, the coating disc is processed to produce operative gratings from the latent images in the photosensitive coating.
In various types of flying spot scanners, cross scan error, unwanted beam deviation perpendicular to the direction of beam scan, causes an undesirable artifact in the written image, termed banding. It is the nonuniform spacing of scan lines that causes the visual appearance of dark and light bands in an image area that would normally be a uniform gray level. Banding related to flying spot scanners comes primarily from two sources: facet-to-facet non-uniformity in reflectivity or diffraction efficiency, and facet-to-facet pointing error. This present invention deals exclusively with the latter. In mirror type scanners (polygons), the mirror facets are not parallel to the axis of rotation causing clustering and spreading of the written scan lines. A similar, but much reduced, problem occurs in hologons where the grating pitch varies slightly from facet-to-facet because of subtle changes that have occurred in the hologon fabrication fixture while the facets are being exposed. The consequence of non-uniform grating pitch from facet-to-facet on the hologon is non-uniform line spacing when the hologon is used in a laser writer.
Depending upon available laser power and photoresist sensitivity, each hologon facet takes many seconds to minutes for exposure. During the time for fabricating an entire disc, thermally induced drift in the laser cavity or in the exposure fixture can cause creep in the laser wavelength or fixture geometry. The result is slow drift in the grating pitch as successive facets are made. This drift can result in a ramp-like variation in cross scan error (FIG. 8) with successive facets around the disc. The error between adjacent facets is small until the large discontinuity between the last facet and the first facet. Such an error can produce noticeable banding because error between adjacent facets has the most effect on local exposure.
A number of methods are known in the art for correcting cross scan error in multifaceted rotating scanners. One method taught by Fleischer in U.S. Pat. No. 3,750,189 was to use combinations of cylindrical and toroidal lens elements to focus the beam on the mirror surface in one dimension and to relay that image to the final plane. This passive technique allows substantially more error in the scanning element, reducing the cost of the element itself, but toroidal lenses are difficult to make and align and the amount of correction afforded is limited in both scan angle and resolution. An active correction system for a hologon scanner such as acousto-optic or electro-optic deflection has been taught in U.S. Pat. No. 4,786,126 to Kramer. Use of such a system adds cost and complexity to the equipment and generally requires measurement and testing of each scanning element. Additional optics are often required to get the beam into and out of the deflection system.
The three patents cited below apply to the fabrication of hologon scanner discs and assume everything is perfect in the laser and exposure fixture and deal with other details of the exposure without altering the sequence of facet exposure as taught by the present invention.
U.S. Pat. No. 4,787,688 to Rumfola teaches a method to improve duty cycle by overlapping facets.
U.S. Pat. No. 4,455,061 describes a "facet" which is made up of a number of small areas of different plane wave (linear) holograms thereby providing a complex scan pattern with the "facet". The hologram is linearly translated, e.g., left to right to accomplish the scan and then must be reset to the original starting position. The reciprocating action required for this is slow, complex and generally suffers from a poor duty cycle when compared to the rotational motion that is more commonly employed. In a laser printer, a fast, linear scan with a large duty cycle is required. Arbitrary scans may be more useful in conjunction with optical memories, laser radars, or target designators. A linear scan could be accomplished using this technique. Because it uses numerically sequential exposure of facets and subfacets, the problem of thermal drift will be the same in the laser and exposure fixture as described previously.
U.S. Pat. No. 4,747,646 rotates a hologon disc mounted on a motor at high speed to gyroscopically stabilize it to eliminate decenter problems. Decenter is a major problem because of a focusing (nonlinear) hologon facet. A hologon made with a linear facet is very insensitive to such error and does not require such an expensive technique for facet exposure. The exposure appears to be sequential and there is nothing in the exposure sequence to compensate for thermal problems.